Visual observations of five radiotracked male weasels Mustela nivalis Linnaeus, 1766 hunting rodents are reported. The weasels hunted bank voles Clethrionomys glareolus and yellow-necked mice Apodemus flavicollis in the primeval deciduous forests of the Białowieża National Park, Eastern Poland. Densities of rodents were high as a consequence of mast crop in the year preceding the observations. A total of 60 attacks by weasels were observed from October till December 1990. Weasels encountered voles and mice at a 1:1 ratio, similar to the ratio these rodents were captured in live traps (1:0.8). Only 23% of weasel attacks on voles, against 60% of those on mice, were successful. Out of 30 attacks on bank voles, 22 were on solitary voles and 8 on social groups of 2 - 4 voles. Bank voles were found by weasels in underground dens and on the ground. Out of 30 attacks on mice, 19 were on solitary individuals and 11 on groups of 2 - 6 mice. Mice were encountered by weasels in underground dens and in cavities located 1 - 4 m up old trees. It is suggested that the mechanism of greater susceptibility of mice to weasel predation in autumn and early winter lies in the physiology of mice, i.e. in their daily torpor. When hunting bank voles, the weasels were more successful with solitary voles than with social gToups. By contrast, attacks on groups of mice were more successful than those on solitary mice. In both species, the rodents in groups benefited from the 'dilution effect' in a group and had a higher probability of surviving weasel attacks than did solitary rodents. The antipredatory defence most frequently observed in both species of rodents was running out of a burrow or cavity when a weasel entered it. Mice often escaped by running away and climbing trees.
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